The most chilling moment came when he held up a vial which could have contained anthrax, to show the size of Iraq's biological weapons programme.

David Kay, a former chief weapons inspector, told US television networks that Mr Powell had made a convincing case.

"He met very well the challenge of talking directly about why we believe this
is a threat," he said.

And Ken Pollack, a former White House adviser on Iraq, said that there was "far more evidence" than Mr Powell was able to lay out.

Political debate

But the presentation may not have changed many minds among politicians.

It was not clear that Mr Powell's case would change many minds among the public

Senator Edward Kennedy, who has been urging the Senate to vote on its own resolution before committing American troops, renewed his warning that the US might be in more danger from terrorism if an invasion went ahead.

And he said that the case was not made linking Iraq to al-Qaeda, noting that al-Qaeda operates in many countries.

However, key Democrat and former vice-presidential candidate Joe Lieberman gave Mr Powell his full backing. And, his rival Senator John Kerry said that "Secretary Powell made a compelling case".

"I don't think any amount of evidence will ever the convince the appeasers that Saddam has to be taken out," he said.

Cuban missile crisis echoes

The early evidence suggests that the speech was not gripping the public in the way that the UN speech of Adlai Stevenson gripped the nation 40 years ago during the Cuban Missile Crisis, when Mr Kennedy's older brother was president.

Then, Mr Stevenson electrified the world with his charge - backed up by photographic evidence - that the Soviet Union had installed medium range missiles in Cuba.

There were fewer people watching Mr Powell live at the UN this time, and for all the worries about terrorism and weapons of mass destruction, the direct threat to the United States seemed far more distant.

The event was not broadcast live on all the main television networks, only on the cable news stations, leaving many Americans to get their interpretation of the testimony from the evening news or the morning newspapers.

And it was not clear that Mr Powell's case would change many minds among the public, many of whom have been debating the Iraq issue for months.

One taxi driver told BBC News Online that he believed the war in Iraq was wrong, and aimed at gaining US control of the oil fields.

A Georgetown student, however, said he was convinced by the evidence that Saddam was a much more serious threat than he had thought.

Even Mr Bush's State of the Union speech last month, which concentrated heavily on the evil nature of Iraq, and was widely publicised, only added a few percentage points to public support for military action against Iraq.

Recent polls suggest that between 53% and 58% of Americans would support that option, but most also favour working through the United Nations.

But there is one element which weighs most heavily with the public.

Over 90% of the public say that if a link between that al-Qaeda and Iraq could be proved, that would justify a war.

So it may be that part of Mr Powell's speech which will prove to be the most important element of his presentation.